The present invention relates to a circuit switch connected between transmitting and receiving devices and in particular to a switch having
a switch memory including a number of memory positions for entering of received time slots from said transmitting devices, PA1 a control memory including a number of memory positions for entering read-out addresses for said switch memory, PA1 an addressing path from said control memory to said switch memory for a read-out address in the latter, from which read-out of data shall be made towards a receiving device and PA1 a control unit for entering said read-out addresses in said control memory and for receiving information from said devices related to the connect and disconnect of calls. PA1 said addressing path from said control memory having an alternative communication path towards said receiving devices, and PA1 each memory position in said control memory including a specific position for indicator bit means containing information with respect to whether read-out of a time slot towards a receiving device should be made from said switch memory or from said control memory.
In switches it is a problem to generate idle codes for disconnected calls. It is important to have a known idle code, e.g. for a processor connected to the switch. If the processor does not have any known idle code it implies in general that the processor cannot distinguish between valid through connected signals and garbage. Moreover it can imply problems to synchronise when a real signal once arrives.
Examples of idle codes are .mu.LAG, aLAG, X25-FLAG, X25-RESET.
A usual method to solve the problem is to switch to a predefined homing position or not to take any action at all in the switch. In the latter case the receiver must use a code of its own as far as required. This will work well enough in the cases where telephone subscribers are connected or when a terminal connected to the switch can generate idle codes for e.g. trunks. In cases where switching is made to a predefined homing position a specific device is required for each requested idle code only for this service.
In U.S. Pat. No. 4,718,062 a telecommunication system is described in which circuit switched data and packet data are communicated. The transmitting device includes a first memory with channels corresponding to respective time slots. Each channel includes status information for indicating whether that channel is in a busy or idle status. Further there is a second memory.
A multiplexor sequentially scans the channels in the first memory within the respective time slots and is dependent on status information in each channel for recovering data from the first memory within a time slot when the status information indicates that a channel is busy and for recovering data from the second memory within a time slot when the status information indicates that the channel is idle.
In EP 250 160 a system is described where circuit switched data and packet data are inserted in different time slots. A memory has an individual position for each time slot. In the memory information is entered which specifies if the individual time slot should serve circuit switched data or packet data. A specific information bit is inserted in each time slot for specifying if the rest of the bits in the time slots should represent one or the other type of data.
None of the above publications is concerned with the generation of idle codes.